By popular request (really, I got quite a few emails about this!...;-), I finally completed my Beogram DC motor restoration video! It demon...
Friday, April 20, 2018
I recently received a Beogram 4002. The owner already had started working on it, but then decided to send it over for some trouble shooting. This shows the unit:
It is in pretty good condition. The aluminum parts are almost pristine except a few small scratches in the platter. The keypad has the usual wear pattern and some blank spots on the START button. I took out the aluminum panels and had a look:
When I plugged it in and pressed start, the platter motor started running but the carriage would not move at all. Pressing start results in turning on of TR12 whose collector then goes to about 17V. This voltage turns on TR19 in the H-bridge that controls the carriage motor. I checked the presence of the 17V at TR19 when pressing start. The voltage made it there indicating that TR12 was o.k. Then I checked the voltages at TR20 when TR19 was activated. This test showed that TR20 would not turn on. I extracted it from the board and indeed a transistor tester revealed that it was burned out. I replaced the transistor with a new one, put the board back in place and voila, pressing start activated the carriage.
However, after moving about 2-3mm out, it immediately returned to the home position as if STOP was pressed immediately after start. I checked the voltage at the base of TR13, which controls the stop function. The voltage there was consistently 0V, indicating that either something pulled the base down to 0V or that TR13 itself had a problem. First I checked the start and end switchers, which can pull the base down, but both switches worked properly. Then I extracted TR13 and the transistor tester showed it to be damaged. I replaced it with a new transistor and then everything worked. Pressing start launched the carriage and it found the set down point for LPs and the arm lowered. Then it started racing towards the center of the platter, indicating that the tracking feedback needed some adjusting. Here is a picture of the two extracted transistors:
This Beogram is on the way to recovery!
Thursday, April 19, 2018
This Beomaster is getting closer to a test run. I completed the capacitor replacement on all of the boards now. There were no real surprises there. I did find a higher than normal number of capacitors that were out of tolerance and quite a few on the border of being out of tolerance. In addition to the capacitor replacement I also replaced some opamp devices and I reflowed the solder joints on all of the board connectors.
Here are the two replacement capacitors on the display board.
Here are the FM and FM Interface boards.
Next, the preamplifier board.
I had seen quite a bit of dirt, grime and rust on this Beomaster in the previous blog posts so I was wary of possible corrosion on the circuit boards. You can see that the six input level trimmers have some sort of grime on the plastic housing.
Sure enough, several of these trimmer knobs would barely turn. I used two types of Deoxit spray on the trimmers. Each trimmer has three slots that expose the inner workings and I was able to flush the grime and get them working smoothly again.
Along with the capacitor replacement there are seven audio opamp devices that get replaced. Beolover just posted a nice description of why we replace these particular opamps. Here is the preamplifier board with the capacitors removed and new eight pin sockets ready for new LF353N opamps.
...and here is the completed preamplifier board.
The Filter & Tone Controls board also has some audio opamps in the signal path that get replaced. There are also a few capacitors to replace. The tone control sliders and switches also received Deoxit treatment so they operate smoothly and have clean contacts.
As I did on the preamplifier board I added sockets for the new opamps. The sockets are nice because it makes it safer to install the new opamps (no solder heat gets applied to the actual device). Having the socket also makes it easy to switch out a different opamp in the future should that become necessary.
The display and microcomputer boards have only two capacitors each (that get replaced). The display board will be revisited later to change out the display segment LEDs and to replace the four indicator lamps.
Here are the two replacement capacitors on the display board.
This is also just a first visit to the microcomputer board. I will come back to it and replace the two oscillators later.
The 22uF (9C85) capacitor on the left has been replaced before. The original capacitor leads were cut and the replacement capacitor soldered to the remaining leads (still soldered to the board). That isn't necessarily bad but I prefer removing the capacitor and properly soldering in a new one.
An important thing on this microcomputer board is that some of the component leads must be soldered to both sides of the board. The negative lead of 9C85 is one of those type of components.
The capacitor replacements on this Beomaster 8000 are now all done. I can reinstall the preamplifier, FM, FM Interface, Filter & Tone Controls boards in the Beomaster chassis now and start connecting their cables.
Wednesday, April 18, 2018
Beomaster 8000: New Signal Path Opamps and Aplifier Performance Characterization with a QA400 Audio Analyzer
Inspired by Sonavor's recent effort to characterize the amplifier performance of a Beomaster 8000 after replacing all signal path opamps, my Australian customer asked that the same would be done to his 8000 before sending it back (after upgrading its circuit with a muting function for the FM section while the Phono input is selected to eliminate crosstalk).
And I am glad he asked. It turns out that it is a great idea to replace the opamps when performing a full restoration of the Beomaster 8000. My measurements yielded a 14 dBV improvement of the THD (Total Harmonic Distortion) performance on the left channel after implementing new opamps.
This shows the preamplifier and input selector board with the original opamps in place:
and after replacing them with new LF353 units (except the phono input, which was replaced with a low noise LM833 type):
I used IC sockets to spare the opamps the stress of soldering. Probably not necessary, but then why not. I did the same for the control panel PCB. This shows it with new LF353s in place:
After putting the control panel back together I performed a series of measurements to see if the amplifier performance would measure up to the values given in the service manual. It turned out that such a comparison is difficult to make due to the inherently different measurement methodology used in the 80s and with the instrumentation that is available today. But I think the measurements below show that this 8000 is now in good shape.
Let the fun begin:
I use a Quant Asylum QA400 audio analyzer for such measurements. Essentially this device yields a Fast Fourier Transform (FFT) of the audio signal that is put into it, and it can perform some measurements (i.e. calculations on the FFT spectrum) based on that data. The measurements include power, total harmonic distortion, signal-to-noise, and frequency response.
Let's have a look at the output spectrum measured with a 0 dBV 1 kHz signal at the Tape 1 input of the Beomaster:
The dBV values essentially give the amplitude ('level') of the signal relative to a standard 1Vrms signal V0. Since level=20*log(V/V0), a signal change of 20dBV corresponds to a 10x change of the amplitude. As an example, if the measured amplitude of the 1kHz signal at the output of the amplifier is 10 Vrms with an input signal of 1 Vrms from the waveform generator, then level=20*log(10/1)=20*1=20 dBV. If the output signal were 100 Vrms, the level gain would be 40 dBV and so on.
Looking at the spectrum above, you probably wondered why the 1 kHz peak is only at about -8 dBV, while the amplifier operated at a volume setting of 5.0, which is close to the maximum output amplitude it can muster. The reason is that the measurement was performed via a voltage divider that was connected as load at the output of the amplifier. The divider was built from two 4 Ohm 50W power resistors and a 0.1 Ohm 3W resistor in series. This shows the setup:
Since the Beomaster is able to produce 100W output power into an 8 Ohm load, the resistors need to be mounted on a heat sink. I used a RF amplifier can that I had laying around from another project. The Y-shaped red wires that are soldered to the small 0.1 Ohm resistor connect to the BNC jack on the right side of the RF can, which then is connected to the QA400 input. The resistor chain is connected on the left to the speaker jack of the Beomaster. This setup guarantees that the QA400 input never gets more than about 1 Vpp. But it also means that the output amplitude that the QA400 'sees' is only 1/81th of the actual amplitude that is applied across the 8.1 Ohms.
Using the above level=20*log(V/V0) formula, we can calculate that the level difference due to the voltage divider is about -38dBV (20*log(1/81)=-38.17 dBV). This means that the 1 kHz peak in the above graph would be at ~-8+38dBV=~+30dBV. I could have shifted the peaks in the graph, but since all measurements that are performed are inherently differences between two levels, this shift really does not matter, i.e. in the following all levels are just as they came out of the QA400, i.e. -38dBV lower than the real signals.
So what do we see in the above spectra? Mainly the THD spectrum to the left of the main 1 kHz peak and some noise. We can see that the 1st harmonic at 2 kHz is about -84 dBV weaker than the main peak. According to the above calculation, this means that the amplitude of that distortion is about 10^4 (= 10,000) times weaker than the main signal. Well below what a human ear could notice. The -84 dBV value is close to the THD measurement result of the QA400, which came in at about -82 dBV (=0.008%). The measurement is a bit worse than the 84 dBV value determined from the graph since there are higher order THD peaks that add to the total distortion. This was the only measurement result that changed before and after replacing the opamps in the signal path. My initial measurements of the left channel yielded a measly -68 dBV THD value, and after the opamp exchange this was improved to -82 dBV. So I think it is a great idea to replace all opamps when the boards are upgraded with new capacitors, just to be on the safe side.
So how does this value compare to the THD value from the Beomaster 8000 service manual? The value stated there is "< 0.05%". So we could be happy and say: "Wow this 8000 is almost 10x better than the value in the manual!". Not so fast, I would think, since the manual states that the measurements were performed according to the "IHF A-202" standard. At this point I do not know what this means, i.e. we need to postpone this comparison with the stated values. But I think we can confidently say that this Beomaster is performing reasonably well and is probably within the original specifications.
Another interesting measurement to perform on an amplifier is its signal to noise (SNR) ratio. This essentially gives us a number that qualifies how much stronger the signal is relative to the noise ("hiss") of the amplifier. This measurement is a bit more difficult to do and understand since we are comparing a defined signal peak amplitude (or power) with a diffuse noise background that is composed of a continuum of frequencies spanning the entire audible range and beyond.
I played a bit with the QA400 settings and it turned out that the SNR measurement is strongly dependent on the number of samples used for the FFT transformation of the input signal. I was able to "change" the SNR value from 65 dBV to 88 dBV simply by changing the FFT resolution from 8196 samples to 65535 samples. This means that depending on the setting the SNR value changed by a factor of 10, one full magnitude. So what is happening? A bit of reading up on the internet and a semi-cryptic response from the Quant Asylum tech support suggested that at lower FFT resolutions the main 1 kHz peak spreads out over several 'frequency bins', thereby lifting the spectral power outside the 1 kHz line, which is misunderstood by the FFT algorithm as part of the noise. This means that at higher FFT resolutions this 'frequency spill out' becomes less pronounced. So we can assume that the higher value is closer to the true value, even though the true value may be even better. Unfortunately the QA400 can only go to 65535 samples, i.e. we would need to find better equipment for an answer.
Wondering about this topic, I performed an experiment that would allow me circumventing the FFT based calculation of the SNR. I measured the signal power of the spectrum at various FFT resolutions and it turned out that the PWR value is not significantly dependent on the resolution. This makes sense since PWR measures the power of the entire spectrum, i.e. integrates over the entire frequency range. So resolution is of limited importance, as long as the signal peak is still 'caught' in one of the sampled frequency bins.
This realization enables a basic SNR measurement: Measure the PWR value with the signal present at the amplifier input and then ground the input and measure again. Subtract the two values and the SNR is obtained.
My measurements yielded on both channels -8 dBV with 1 kHz signal (at Volume setting 5.0) and -98 dBV with the input grounded (also at Volume 5.0). The difference is 90 dBV, a bit better than the best 88 dBV value measured via FFT analysis. This compares to a ">77 dB" stated in the service manual. Again, we do not really know at this point how the 77 dB value was measured, but I think we are on the safe side and can conclude that this Beomaster 8000 is operating on spec.
Another interesting measurement is the frequency response ("FR"). The QA400 does this measurement by sending a square pulse into the input of the amplifier and then measuring the response at the output. Since a square pulse contains all wavelengths, the FFT of the response yields a true spectrum of the amplifier FR. I verified this by manually measuring the transmission for a few frequencies and the FR curves were exactly matched, i.e. I think we can believe this measurement as it comes out of the QA400. This shows the FR spectrum measured on both channels:
I cut the spectrum off at 1 kHz since there was some 60 Hz noise, and the FR drop at 20 kHz seems typically referenced to 1 kHz. So what we see from the graph is that there is a 1 dBV drop from 1 kHz to 20 kHz. The manual prescribes 0.5 dBV, but of course, again, we do not know how this was measured etc...A difference of 0.5 dBV corresponds to a signal ratio of 10^(0.5/20)=1.06, i.e. the signal at 20 kHz is 6% smaller than it should be. Not very dramatic, and most likely this discrepancy is a result of the different measurement methodologies that were applied in the 80s.
Since everything was hooked up, I decided to measure the FRs for the various filters the 8000 has. This shows the spectra:
The spectra show the FR for filter 1, 2 and both active, for bass and treble sliders set to minimum and maximum, and the flat (filter button "off") response in direct comparison. We can see nicely that the treble and bass sliders allow a ~±10 dBV change of the higher and lower frequency ranges, and that the filters cut off around 7 kHz and 10 kHz at as prescribed in the manual.
So in summary, I think we can say that this Beomaster is in excellent condition and that everything works as it should.
Monday, April 16, 2018
The key panel of the Beomaster 6000 quad is an interesting design that can also be found on the Beogram 4002/4004/6000 (not on the BG4000), the Beocord 5000 (type 47XX) and even to a certain extent, on the Beomaster Commander.
Little information is available on the construction of this key panel, so I made a small drawing of all the elements that make up this key panel.
The actual contact is made by pushing on the metal "tongues". Under these tongues is a locking plate (in black plastic) that is glued under the key panel. This locking plate then pushes the small purple plunger against a copper plate that makes electric contact with the copper bridge in the switch. Quite a long journey to make a contact !!
The metal key panel is made of stainless steel spring steel. It is brushed on one side (the visible side) in an industrial way with stainless steel brush rollers with a grid of 400-600. I have tried to replicate this brushed aspect on another key panel that had been sanded by someone, but did not succeed. It is impossible to get perfect straight brushed lines into the metal without using the proper industrial equipment.
The steel plate is also coated on top to give it a more satin/matt aspect in stead of the bright shiny steel. This coating is often gone on the keys that are mostly used. That creates the typical finger prints that are seen. The grease of the skin affects the coating over time.
It is also interesting to see that the steel plate tends to bend upwards if the glue is dried up after several decades and releases the steel plate. There is reason for this! The steel plate is made out of a flat piece of metal, but is then rolled to get a certain curve (see picture). This is done to create a pre-tension on the "tongues" and to make sure that the locking plates firmly touch the underside of the aluminium frame in rest. By doing this, the tongues should be perfectly flat with the rest of the panel and it also assures that no false contact is made in the electric switches.
After this rather long explanation, let's start to take the panel apart. First thing to do is to remove the locking plates. If not, you can not remove the key panel from the frame. On this particular unit, the locking plates were not only glued to the steel plate but some epoxy glue was added to make sure they stay in place. In fact, it's not a bad idea to do this since I recently got a restored Beomaster 6000 back in my work shop for a repair after a damage caused by a power spike. I noted 2 of the locking plates falling of again.
Anyhow, I had to remove the epoxy glue first from all of the 20 locking plates:
After this, the best way to remove the locking plates completely is by given them a short, firm knock with a hammer using a piece of wood. If you try to prime them off (like with a screwdriver), there is a high risk that you damage the key panel. It leaves marks on the top, visible, side of the panel.
The locking plates always have there small pieces of plastic still attached from the injection machine tools. Not really Beolove.....So, I always cut those pieces of.
Once all the locking plates are removed, it's time to get the steel plate detached from the aluminium frame. Most of the time, this is fairly easy done since the glue used at the time did not hold very well. I use a long cutter knife that I stick between the plate and frame. On the Beograms and certainly on the Beocords they seem to have used another type of glue that holds very well (and very difficult to remove...!).
The hardest part is now to get all the old glue removed. I use a flat cutter knife again and some fine grid sandpaper. Followed by an alcohol cleaning.
The most difficult part is coming up now: to get the key panel recoated! As mentioned, the panel is brushed with some coating on top. Those fingerprints are in fact areas where the coating has disappeared over time. Small scratches in the coating do not leave permanent marks. Once the metal itself has scratches, they can not be removed. DO NOT TRY TO SAND THE PANEL. It will ruin the brushed, matt aspect forever....
So, let's remove the coating. A special paint removal (used in the automotive industry) is used.
I always need to apply a few layers of paint removal before all the coating is gone. The chemicals used in the paint removal are dangerous, so take the necessary precautions. They act very fast (10- 30 seconds). A good wash with water and detergent is next. And yes, they shine now........and that is not the way we want it!
We need to bring back the coating. Another difficult task. I've tried different coatings/lackers and again the best is matt/satin clear lacker used in the automotive industry. You need a dust free environment to apply the coating. I looked at how spraying cabins for cars are made and created my own, miniature spraying cabin. It's made out of wood, with a plexi hinged door and a Durst UT100 on top. This Durst is used by analog film photographers to dry there rolls of film pelicule. It has a blower, a heater (with 2 settings) and a dust filter build in. It blows into the cabinet providing a small overpressure with the idea to keep dust out.
It works fairly well, but still not easy to keep all dust particle out.
Applying the coating requires some experience. I have done this like 8 times now, and still make mistakes. This time I put to much coating on the plate the first time and had to redo the procedure of cleaning and coating all over again.
The coating also requires 24 hours of hardening. I keep the heater on during the first couple of hours on setting 1 (about 40 degr. Celcius).
This is the result once the whole proces is done. The matt, satin look is back again !
Not bad at all I must say !
Back to the aluminum frame now. Once all the glue is removed from the aluminum frame, it's time to get the black lines repainted on the frame. Some masking tape is used and a fine brush to paint the lines back with matt black metal paint.
This looks a lot better already !
Once the frame is cleaned and the lines repainted, the steel panel plate needs to be glued back on the frame. To make sure that both pieces are perfectly aligned, I made a wood fitting piece. It's important that the gap (1mm) between the key "tongues" and the aluminium frame is correct and the same all along the frame. I use 4 metal plates of exactly 1mm thick as chims.
With the two pieces well aligned and the glue added, I use a wood plank and spanners to hold everything together until the glue has dried.
While waiting for the glue to dry, I took the FM dial wheel/flywheel spindle through the same procedure.
And 24 hours later, time to check all is OK...
Looks like new (well, almost...)!
All the black plastic locking plates are back in place as well.
And a few more close-ups
We are now getting really close to the finish of this Beomaster 6000 quad restoration. In my next post I'll explain the polishing of the red plexi display panel.